There appears to be a healthy competition developing between optical and electrical communication systems. If electrical systems are to remain viable for distributing signals at high transmission speeds, then electrical cables and connectors must improve their transmission performance or face replacement by optical systems. However, since nearly all consumer and business communication systems are equipped to handle electrical signals exclusively, electrical systems presently enjoy a competitive advantage. Nevertheless, the replacement of electrical equipment with optical equipment may ultimately occur anyway, but it can be forestalled for the foreseeable future by substantial performance improvements. Compared to optical cables, electrical cables suffer from limited broadband capability and have greater crosstalk susceptibility. One of the most efficient and widely used electrical cables, which has both broadband capability and immunity from crosstalk interference, is the well-known coaxial cable.
Coaxial cable was invented at Bell Laboratories on or before May 23, 1929 by Lloyd Espenschied and Herman Affel (see U.S. Pat. No. 1,835,03 1), and it seems unlikely after so many years that it might still be possible to improve its performance in any meaningful manner. Nevertheless, such improvement is sought.
Coaxial cable comprises an electrical conductor (hereinafter "inner" conductor) that is completely encircled by another electrical conductor (hereinafter "outer" conductor) with a non-conducting layer between them. The thickness of this layer is, ideally, uniform and may comprise air, but most often comprises a dielectric material such as polyethylene. Coaxial cables transmit energy in the TEM (Transverse Electromagnetic) mode, and have a cutoff-frequency of zero. In addition, it comprises a two-conductor transmission line having a wave impedance and propagation constant of an unbounded dielectric, and the phase velocity of the energy is equal to the velocity of light in an unbounded dielectric. Coaxial cable has other advantages that make it particularly suited for efficient operation in the HF (High Frequency) and UHF (Ultra High Frequency) regions of the electromagnetic spectrum. It is a perfectly shielded line and has a minimum of radiation loss. It may be made with a braided outer conductor for increased flexibility, and it is generally impervious to weather. Inasmuch as the coaxial cable has little radiation loss, nearby metallic objects and electromagnetic energy sources have minimum effect on the cable as the outer conductor serves as a shield for the inner conductor.
Asymmetrical imperfections such a ovality of the dielectric material, out-of-roundness (eccentricity) of the wire cross section, and lack of perfect centering of the wire within the dielectric material tend to limit the high-frequency performance of coaxial cables. These imperfections are practically unavoidable during manufacture for a variety of reasons including: tool wear, gravity, unequal flow of dielectric material during extrusion, tolerances, etc. As a result of such asymmetrical imperfections, a variety of transmission problems can arise including signal reflections (i.e., structural return loss), distortion, and loss of power. Variations in the electrical impedance of the coaxial cable at different points along its length, caused by minor changes in the distance between the inner and outer conductors, give rise to signal reflections. Such reflections shorten the distance that a signal can be transmitted along the coaxial cable without error, and limits the maximum frequency that can be supported.
In an attempt to improve the SRL (Structural Return Loss) performance of a coaxial cable, manufactures have employed a variety of different schemes focusing on concentricity and eccentricity of the central metallic conductor within the dielectric insulation. These schemes have not yet yielded sufficient improvement in a practical manufacturing environment and, accordingly, new techniques for improving SRL are desirable.